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Homology vs. Analogy
Not all shared characteristics are created equal.
Biologists rely on the presence
of shared characteristics when constructing phylogenies. These characteristics
come in a wide variety of forms - anatomical or morphological similarities,
behavioral similarities, similar biochemistry, similar DNA, and so on.
However, it is important to realize that some shared features are more
useful than others in constructing phylogenies that reflect the true evolutionary
relationships among taxa.
Shared characteristics can be classified into two groups, homologous
characters and analagous characters. The difference between these kinds
of characters has to do with the particular way in which features are
similar.
Features are homologous when they can be traced back
in time to the same feature in a common ancestor. Because of this they
are important
evidence for building phylogenies. Another way to think about it is to
say that homology means similarity due to shared ancestry. For example,
although they appear to be quite different
superficially, at the level of skeletal structure the wing of a bat,
the forelimb of a mole, and the front flipper
of
a dugong
are built of the same parts, modified to perform different functions (see
image at right).
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In contrast, analogous characters
are similar because they perform similar functions. They may or may not
share evolutionary origin. As products of convergent evolution, analagous
characters are not useful in reconstructing phylogenies. Simply put,
organisms evolving on earth are subject to the same
sets of physical laws. This means that when two species are put under similar
selective pressures natural selection may produce sometimes strikingly similar
solutions despite the fact that the species in question may be relatively distantly
related evolutionarily.
For example, a wing, regardless of what materials it is built from must
have certain characteristics to be effective. For this reason, we see convergence
in the wings of bats, birds, and extinct pterodactyls, not to mention the
wings of insects and flying fish! |
 
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Homologous characters can be further classified as either primitive
or derived. A primitive character
is one that has the same condition as the common ancestor of the compared
groups. For example, hair would be considered a primitive character
of mammals because it is a feature that is thought to be present in
all mammals because it was a feature of the common ancestor of all
mammals. Likewise, DNA could be considered a primitive characteristic
of all organisms on earth. The presence of DNA in all living things
suggests that all life shares a common ancestor.
In contrast, derived characters are those that
differ in condition from the common ancestor. For example, the water vascular
system of echinoderms is a feature that is common to all echinoderms but is
not found in other deuterostome groups. Shared derived characters (often called
synapomorphies) are useful in defining groups when constructing a phylogenetic
tree. Again, the presence of a water vascular system, for example, can be used
to "diagnose" an organism as an echinoderm.
Note that homology and analogy and primitive
and derived are relative terms. This means that a feature that is considered
derived at one level of a phylogeny may be considered ancestral at another
level. An example will illustrate this point. When we look at vertebrates as
a group, the presence of vertebrae is considered to be a primitive character,
something that was present in the common ancestor of all vertebrates. However,
when we look at deuterostomes (the larger clade that includes the vertebrates),
the presence of vertebrae would be considered a derived character as it is
not thought to have been present in the common ancestor of all deuterostomes.
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